Mold release agent and method of application for die casting

ABSTRACT

A die cast mold release agent comprises a lubricant selected from at least one of the group of silicones, olefins, vegetable oils, waxes and organic esters of fatty acids preferably very little water. A method is of application of The pressure die cast mold release agent is disclosed where a die casting machine has a mold with mold halves forming a mold cavity when the mold halves abut against one another, a shot sleeve and a plunger which pushes molten material through the shot sleeve and into the mold cavity. The mold release agent is placed into the pressure die casting machine remote from the mold halves, molten material is introduced into the shot sleeve, and the molten material and the mold release agent are forced into the mold cavity by movement of the plunger into the shot sleeve.

FIELD OF THE INVENTION

[0001] This invention relates to improvements to pressure die casting mold release agents, and to an improved method of application of the mold release agent in the pressure die casting process.

BACKGROUND OF THE INVENTION

[0002] Pressure die casting is a well known process in which molten metal is forced under pressure and heat into a mold cavity of a mold of a die casting machine. The molten metal is introduced in a shot sleeve, and a plunger forces the metal into the mold cavity through a shot sleeve. While in the mold cavity the molten metal solidifies resulting in a molded part having a geometry conforming to that of the mold cavity. After the part is formed, it is then ejected from the mold cavity. Molded parts so produced have excellent dimensional tolerances, structural integrity and cosmetics.

[0003] There are at least two distinct classes of lubricants used in pressure die casting, each applied to a different area of the pressure die casting machine. One is a plunger lubricant, used for the “shot end” of the die casting machine; the other is a mold release lubricant, used for the mold cavity and adjacent area of the die casting machine. At the shot end of the die cast machine there are tight tolerances between the plunger tip and the shot sleeve. Friction may interfere with the efficient movement of the plunger through the shot sleeve. The plunger lubricant is applied to the plunger tip or immediately adjacent the plunger to provide hydrodynamic and boundary lubrication between the plunger and the shot sleeve and thereby enhance smooth travel and acceleration of the plunger through the shot sleeve. Plunger lubricants also provide a barrier film between the molten metal and the plunger to help prevent adhesion of the metal to the plunger and to help reduce washout of the shot sleeve.

[0004] While solidifying in the mold cavity, the molten metal will have an affinity for the surfaces of the steel mold halves. This can result in stuck castings, soldering or other damage to the molded parts. To help avoid these problems, the second class of lubricants, mold release lubricants, have been used to help ensure proper removal of the cast part from the mold cavity, and to lubricate parts in the mold cavity as well as any runner and gate areas of the mold. Mold release agents should be thermally stable at high temperatures (as they are routinely exposed to molten metal), should have good wetting properties, should provide a barrier release between the cast part and the mold cavity, and should enhance molten metal flow.

[0005] Therefore, in all known pressure die casting processes the mold release agent is required to lubricate the mold halves for proper release of the cast part, and the plunger lubricant is required to lubricate the plunger as it travels in the shot sleeve. Known plunger lubricants have been liquids comprised largely of a mineral oil and other organic or inorganic materials added to enhance hydrodynamics and/or boundary lubrication and to reduce friction between sliding surfaces. Materials used in plunger lubricants are noted for providing lubrication between moving surfaces, and have not been noted for providing the thermally stable function required of mold release agents.

[0006] U.S. Pat. No. 5,076,339 to Smith discloses a plunger lubricant formed as a solid, wherein the plunger lubricant comprises at least 40% wax. The solid wax lubricant is applied through the pour hole into a shot sleeve at the plunger. This solid plunger lubricant has found some commercial success for lubrication of the shot sleeve and plunger tip. However, a mold release lubricant is still required to be applied to the mold halves, just as with all other heretofore known plunger lubricants.

[0007] U.S. Pat. No. 5,039,435 to Hanano discloses a pressure die-cast mold release agent formed as a powder, essentially a solid lubricant. It comprises three elements: a lubricant, an organic polymer, and a metal soap. The lubricant can be one from the group of boron nitride, silicon nitride, molybdenum disulfide, graphite, mica and metal oxides (inorganic materials). The organic polymer can be one of eight different materials: polyethylene, silicon resin, polypropylene, phenol resin, polystyrene, acrylate resin, epoxy resin and alkyd resin. The lubricant is coated with the organic polymer or the metal soap. The powdery mold release agent is applied to the mold cavity halves electrostatically and in a vacuum, requiring incorporation of expensive capital equipment.

[0008] The September-October 1972 issue of Die-Casting Engineer discloses an article by Radtke which teaches a pore-free die casting technique. Here, air is drawn out of the mold cavity and replaced with oxygen. A solid lubricant is applied to not only the mold cavity, but also the plunger tip and to the shot sleeve. The composition of the lubricant is not defined, but it does state that “oily type lubricants cannot be used in the process.” Petroleum oil based mold release lubricants (i.e., “oily type lubricants”) have been used in the past, but have fell out of favor due to safety and environmental concerns.

[0009] Currently, almost all mold release agents for pressure die cast manufacturing are liquids. More specifically, conventional mold release agents are water-based emulsions which may contain waxes, oils, silicones, surfactants, anti-foams, extreme pressure additives, rust inhibitors, and biocides. The mold release agent is combined with air under high pressure and then directly applied onto the mold cavity through spray nozzles. Typically water comprises about 45-90% by weight of the composition of conventional concentrated mold release agents. In turn, the concentrated mold release agent is then diluted with additional water immediately prior to application to the mold halves. These water-based mold release agents have the advantage of good heat extraction and lubrication properties. However, the large amount of water creates enormous amounts of waste effluent which must be cleaned from the machine and which must be removed and treated at additional expense. Also, large amounts of water cause thermal cycling of the mold halves, significantly shortening mold life. In addition, uneven application of mold release agent can result in uneven temperatures across the mold, which may deleteriously affect part quality. It would be desirable to have a mold release agent with little or no water while still maintaining good heat extraction and lubrication properties.

SUMMARY OF THE INVENTION

[0010] In accordance with a first aspect, a pressure die cast mold release agent comprises a lubricant selected from at least one of the group of silicones, olefins, vegetable oils, waxes and organic esters of fatty acids preferably very little water. In accordance with another aspect, the pressure die cast mold release agent can comprise a combination of a lubricant selected from at least one of the group of olefins, vegetable oils, organic esters of fatty acids and waxes; and a significant percentage of a silicone. In accordance with another aspect, the pressure die cast mold release agent comprises an encapsulated lubricant comprising a plurality of beads, wherein each bead has a solid exterior and a liquid interior. In accordance with another aspect, a method is provided where a pressure die casting machine has a mold with mold halves forming a mold cavity when the mold halves abut against one another, a shot sleeve and a plunger which pushes molten material through the shot sleeve and into the mold cavity. A mold release agent is placed into the die casting machine remote from the mold halves and remote from the pour hole, molten material is introduced into the shot sleeve, and the molten material and the mold release agent is forced into the mold cavity by movement of the plunger into the shot sleeve.

[0011] From the foregoing disclosure and the following more detailed description of various preferred embodiments it will be apparent to those skilled in the art that the present invention provides a significant advance in the technology and mold release agents for die casting. Particularly significant in this regard is the potential the invention affords for providing a high quality mold release agent which is environmentally friendly. Additional features and advantages of various preferred embodiments will be better understood in view of the detailed description provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic sectional view of a pressure die casting machine in which a mold release agent is used according to a preferred embodiment.

[0013]FIG. 2 is a schematic view of a pair of mold halves, showing the mold cavity, runners and inlet gate.

[0014] It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the method of application of as disclosed here, including, for example, the specific dimensions of the mold halves, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to facilitate visualization and clear understanding. In particular, thin features may be thickened, for example, for clarity of illustration. All references to direction and position, unless otherwise indicated, refer to the orientation illustrated in the drawings.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

[0015] It will be apparent to those skilled in the art, that is, to those who have knowledge or experience in this area of technology, that many uses and design variations are possible for the mold release agent and method of application of the mold release agent disclosed here. The following detailed discussion of various alternative and preferred features and embodiments will illustrate the general principles of the invention with reference to a lubricant suitable for use with die-cast pressure molding of metal parts. Other embodiments suitable for other applications will be apparent to those skilled in the art given the benefit of this disclosure.

[0016] Referring now to the drawings, FIGS. 1 & 2 show part of a pressure die casting machine 8 suitable for forcing molten metal under some pressure and heat into a mold cavity 16 of a mold 10. The mold cavity is formed when a pair of mold halves 12, 14 abut against one another, and cooperate to form the shape of the part to be made from the molten metal. The molten metal is introduced in a shot sleeve 22 through, for example, pour hole 28, and a plunger 24 forces the metal into the mold cavity 16 through a shot sleeve. The molten metal may travel through a runner 18 and/or a gate inlet 20, as shown in FIG. 2, or in some preferred embodiments, the runner may not be used. The gate inlet 20, shot sleeve 22, plunger 24 and any runner(s) 20 are positioned outside of the mold cavity 16, and do not define the geometry of the part. Typically for the die casting process disclosed herein no vacuum is drawn and the mold cavity is not pressurized prior to introduction of the molten metal. Vents 30 may be provided to allow atmospheric air and entrapped gases to escape from the mold cavity 16.

[0017] Heretofore it has been thought by those in the art of pressure die casting that for die casting machines to operate properly, a plunger lubricant needed to be applied (either at pour hole 28 or immediately behind the plunger 29), and a mold release lubricant must be applied directly onto the surface to be lubricated. However, in accordance with a highly advantageous feature, the inventors have determined that a mold release agent of some compositions may be applied to the gate, the runner, or to the shot sleeve, or to any combination therewith, (i.e., remote from the mold halves) and yet still function properly to lubricate the mold halves and permit release of the cast part from the mold cavity. Essentially, the metal pushes the mold release agent in front of it, forming a lubrication front that coats the mold halves as it travels through the mold cavity. This method of application advantageously can reduce the total amount of mold release agent that needs to be applied, reducing costs for the die caster. Additional savings may be realized by reducing waste effluent. Further cost reductions may be obtained through shorter cycle times since less lubricant needs to be applied and the mold release of the present invention may be applied more rapidly than known mold release agents.

[0018] The lubricant used in the mold release agents disclosed herein can advantageously be one of many types of low friction materials providing good thermal stability endurance and having either solid, liquid or wax-like properties and good wetting and adhesion properties. Lubricants suitable for use herein can comprise, for example, silicones, polyolefins, organic esters, and waxes. Synthetic oils made of one or more of these materials may be used. Also, some natural oils may be used as all or part of the lubricant, such as vegetable oils including rapeseed oil, castor oil and soybean oil. Such suitable materials may be used individually, or mixed together with other materials to form the lubricant suitable for use as a mold release agent. The die cast mold release agents may also optionally be formed as a solid or flexible gelatin-like material encapsulating a liquid and applied as a series of beads.

[0019] Silicones comprise a wide variety of polymers that have alternating silicon and oxygen atoms in repeating diorganosiloxy groups. Such silicones may also incorporate one or more organofunctional groups. Examples of suitable materials are Dow Corning's DC-203 and 230 fluids, DC-2503 silicone and DC-2-5088 wax. Olefins generally comprise any unsaturated aliphatic hydrocarbon with one or more double bonds, and two hydrogens for every carbon (e.g., polymerized ethylene or butene). Olefins can comprise polyalphaolefins, alpha olefins and normal alphaolefins and polybutenes. Polyolefins are often a large component of synthetic oils. Examples of suitable materials are Petro-Canada API Group III and IV Oils; Durasyn 162 supplied by Amoco; Synton PAO 100 supplied by Crompton Corporation; Indopol H-100 by BP Amoco. Organic esters suitable for use as a lubricant here can comprise, for example, sucrose esters of fatty acids such as Sefose® by Proctor & Gamble, carboxylic acid esters, methyl esters such as Keil's Base ML, methyl canola esters and methyl tallowate esters. Waxes can comprise a wide variety of materials including natural waxes such as paraffin and other petroleum waxes, lignite, peat and montan waxes, animal waxes, and vegetable waxes, or synthetic or partially synthetic waxes such as polyolefin waxes, polar synthetic waxes, and Firscher-Tropsch waxes. Suitable waxes often at least partially comprise carboxylic acid esters, oxidized polyethylenes, ethylene acrylic acid copolymers, bis-stearamide wax or microcrystalline wax. Other lubricants suitable for use as all or part of a mold release agent will be readily apparent to those skilled in the art given the benefit of this disclosure.

[0020] As there is no need to spray this mold release agent over the mold halves 14, 16, the mold release agent can comprise a significantly small amount of water compared with current known mold release agents. Preferably the mold release agent comprises less than 20% water by weight, and most preferably no water. For example, the mold release agent can be comprised of essentially entirely silicone with no water, formed either as a solid or as a liquid, advantageously eliminating the need to add water to a lubricant. Alternatively, the mold release agent could comprise a high percentage of wax (e.g., greater than 40% by weight) or one of the other suitable lubricant materials mentioned above and a small percentage of silicone, such as 5-10 percent by weight.

[0021] Preferably the mold release agent would be applied remote from the pour hole in the area of the shot sleeve 35 shown in FIG. 2, and most preferably immediately adjacent the runner 18 as shown by arrow 36 in FIGS. 1-2. It has been found that such application positions are most effective in lubricating the mold halves 12, 14 so that the part created can be properly released from the mold cavity 16. Depending on the intended application, the mold release agent application location can be the shot sleeve, pour hole, and/or any runner and gate inlet connecting the shot sleeve to the mold cavity that is remote from the plunger.

[0022] From the foregoing disclosure and detailed description of certain preferred embodiments, it will be apparent that various modifications, additions and other alternative embodiments are possible without departing from the true scope and spirit of the invention. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled. 

What is claimed is:
 1. A pressure die cast mold release agent comprising, in combination: a lubricant selected from at least one of the group of silicones, olefins, vegetable oils, waxes and organic esters of fatty acids.
 2. The die cast mold release agent of claim 1 wherein the lubricant comprises at least 71 percent by weight of at least one of the group of silicones, olefins, vegetable oils, waxes and organic esters of fatty acids.
 3. The pressure die cast mold release agent of claim 1 wherein the lubricant comprises greater than 80% by weight silicone.
 4. The pressure die cast mold release agent of claim 1 wherein the mold release agent comprises less than 5% by weight of water.
 5. The pressure die cast mold release agent of claim 1 wherein the lubricant comprises one of a liquid, a solid, and an encapsulated liquid having a solid exterior.
 6. A die cast mold release agent comprising, in combination: a lubricant selected from at least one of the group of silicones, olefins, vegetable oils and organic esters of fatty acids; wherein the mold release agent comprises less than 19 percent by weight of water.
 7. A die cast mold release agent comprising, in combination: greater than 40% by weight of a lubricant selected from at least one of the group of olefins, vegetable oils, organic esters of fatty acids and waxes; and greater than 5% by weight of a silicone.
 8. The pressure die cast mold release agent of claim 7 further comprising less than 19% water.
 9. A die cast mold release agent comprising, in combination: an encapsulated lubricant comprising a plurality of beads, wherein each bead has a solid exterior and a liquid interior.
 10. The pressure die cast mold release agent of claim 9 wherein the encapsulated lubricant is selected from at least one of the group of silicones, olefins, vegetable oils, waxes and organic esters of fatty acids.
 11. A die casting method for a die casting machine having a mold with mold halves forming a mold cavity when the mold halves abut against one another, a shot sleeve with a pour hole to receive molten material, and a plunger which pushes molten material through the shot sleeve and into the mold cavity comprising, in combination, the steps of: placing a mold release agent into the pressure die casting machine remote from the mold halves and remote from the pour hole; introducing molten material into the shot sleeve; and forcing the molten material and the mold release agent into the mold cavity by movement of the plunger into the shot sleeve.
 12. The method of claim 11 further comprising the step of applying the mold release agent to a runner connecting the mold cavity to the shot sleeve.
 13. The method of claim 11 further comprising the step of applying the mold release agent to a gate inlet connected to the mold cavity.
 14. The method of claim 11 further comprising the step of applying the mold release agent at the shot sleeve.
 15. The method of claim 11 wherein the mold release agent is applied into the die casting machine in the presence of air.
 16. The method of claim 11 wherein the mold release agent comprises a lubricant selected from at least one of the group of silicones, olefins, vegetable oils, waxes and organic esters of fatty acids.
 17. The method of claim 11 wherein the mold release agent comprises one of a liquid, a solid, and a bead having a solid exterior and liquid interior.
 18. The method of claim 11 further comprising a runner connecting the mold cavity to the shot sleeve; wherein the molten material is introduced adjacent the runner. 